Basically, seismic source signatures are a representation of the acoustic pressure as a function of time. It is characterized as a wavelet that propagates with its free-surface ghost and whose amplitude is inversely proportional to its distance from the source.
Interest in air-gun signatures has included proposals for better measurement methods, empirical investigations of the relationship between signature characteristics and gun parameters, methods for improving synthetic calculations, as well as studies on the effect of the directional dependence of signatures on seismic data.
Moreover, some methods in the literature propose to deconvolve the source signature from marine seismic reflection data taking into account the directivity, for example, Krail, P. M. and Shin Y, in "Deconvolution of a directional marine source", Geophysics vol. 55, no. 12, p. 1542-1548 (1990) and Roberts, G. A and Goulty, N. R. in "Directional deconvolution of Marine Seismic Reflection Data: North Sea Example", Geophysical Prospecting, vol 38, p. 881-888 (1990).
For the calculation of deconvolution filters, those methods require the knowledge of the far-field source signatures radiated in all directions, known as "multidirectional far-field signatures".
The marine seismic sources for oil exploration are air-gun arrays. The arrays are composed of air-guns with different volumes placed at different positions in x- and y-directions but having approximately the same depth. The combination of air-guns with different volumes reduces the bubble effect generated by this type of source placed under water and also enhances the power of this type of source.
The signature of an air-gun array depends on many factors: the range of gun volumes, the array geometry, the gun-firing synchronization, the direction of observation relative to the array, and mainly on all the equipment involved in the acquisition of the signature.
The problem of obtaining the far-field signature is well understood. In the case of source arrays, a source-receiver separation constraint should be considered. A signal generated by the most distant source element in the array must arrive at any point receiver simultaneously with those signals generated by the closest source element of the center of the array.
Such separation is known as the far-field of an array. If the receiver is too close to the array, this requirement is not satisfied and distortion occurs depending on the dimension of the source array. At the far-field the waveform of the signature will vary, according to the position from which it is observed. This effect is called directivity or multidirectional effect, which causes the signature to vary according to the incident angle, and azimuth angles related to the free surface.
The multidirectional far-field signature of an array can not be monitored during conventional data acquisition. R. C. Johnston et al. in Special Report of the SEG technical Standards Committee, SEG standards for specifying marine seismic energy sources, Geophysics, vol. 53 no. 4, p. 566-575 (1988) describe a method using sonobuoys to acquire the signatures. Differences in the survey environment and the instruments used cause significant variation between the true signature and the acquired one.
Therefore a method has been suggested for computing air-gun signatures using near-field hydrophones, and then processing those data to extract the far-field signatures, as described by G. E. Parkes et al. in "The signature of an air-gun array: Computation from near-field measurements including interactions--Practical Considerations", Society of Exploration Geophysics (SEG), vol. 48, no. 2, p. 105-111 (1984). This method appears feasible in practice, but it does add more complexity to the deployment of an air-gun array.
There have also been suggested methods to model the far-field signature. In this case, the important parameters to know are: initial pressure, gun positions, gun volumes, gun firing times, and the open port area Vs time for each gun. Most of those parameters vary from shot to shot and their measurement is not easily obtained during a conventional operation. In this respect, see Dragoset, W. in "A comprehensive Method for Evaluating the Design of Air-guns and Air-gun Arrays". The leading Edge of Exploration, vol. 3, p. 52-61 (1984).
In short, directivity or multidirectional effect arises because of the dimension of the seismic source arrays. The arrays contain air-guns of different volumes to enhance the power of the source and to cancel the bubble effect caused by this type of source as well. It follows that the recorded wave field is the linear superposition of the responses from each source element. This means that spatial smearing should occur due to the finite dimensions of the source array. The smearing varies according to the angle of the wave arriving at the punctual observer, this leading to directivity.
The far-field signature can not be acquired simultaneously to the conventional data acquisition operation. The receivers inside the streamer are not in the far-field because they are behind the seismic vessel at depths varying from 10 to 12 m. The normal procedure is to carry out another survey to obtain a quantitative estimation of the signature, using sonobuoys.
U.S. Pat. No. 4,476,550 teaches a method for ascertaining the far field signature of an array of sound source units, each of which is small, compared with the wavelength of the highest frequency of interest. This is achieved by firing air-guns sequentially so that each generates all its significant radiation before the next is fired, and/or by firing more than one air-gun at a time, and separating them by at least one wavelength of the lowest frequency of interest. The far-field signature of each unit is measured by a pressure-sensitive detector close to the air-gun but in a region where the phase spectrum of the pressure field is independent of azimuth and range. The far-field signature of the array is derived from the measured signature by summation. This method employs the conventional streamer acquisition scheme.
Another experiment to obtain the far-field signature causes some problems of repeatability in some acquisition parameters. Such problems are related to differences in instruments and hydrophone responses, depths of the source, and sea conditions, which may happen when far-field signature and the data are acquired. Therefore a method has been suggested for computing air-guns signatures using near-field hydrophones, and then processing those data to extract far-field signatures, as taught by Ziolkowski, A et al. in "The Signature of an Air-gun array--Computation from near field measurements including interactions" --Geophysics vol. 47 (10), p. 1413-1421 (1982) as well as the above-cited article by G. E. Parkes.
U.S. Pat. No. 4,658,384 teaches a method for determining the far-field signature of an air-gun array by deriving from near-field measurements. An array of air-gun is deployed in the water at a desired depth. A receiver is suspended in the middle of the array at the same depth so that the guns are equidistant from the receiver. The lateral spacing between the guns and the sensor is much less than the water depth of the guns. Having fired the guns, the ghost reflection amplitude in the near field will be much less than the amplitude of the direct arrivals and can be ignored. The far-field is determined by inverting the observed pressure signature, delaying it in proportion to array depth and adding the inverted, delayed signature back to the original signature.
These methods appear feasible in practice, but they do add more complexity to the deployment of an air-gun array.
There have also been suggested methods to model the far-field signature. The important parameters to know are: initial pressure, gun positions, gun volumes, gun firing times, and the open port area vs. time for each gun. Most of those parameters vary from shot to shot and are not easily measured during the conventional operation. Moreover, the modeled signature lacks the recording instrument and other environment effects present in the seismic data.
The conventional seismic processing sequence uses spike deconvolution to remove the wavelet emitted by the source of the seismic data.
The spike deconvolution is based on the use of the Toeplitz method, which requires that the initial function be a minimum-phase, but this is not the case with the air-gun array signature function. The air-gun array signature is close to minimum-phase as a function of continuous time. This phase spectrum is different, however, from the minimum-phase spectrum that is estimated by spiking deconvolution for a sampled and time-windowed version of the signature. As a consequence, large phase errors may arise when spiking deconvolution is applied to an air-gun signature or to a recording instrument response. Therefore, deconvolution of the signature should be used to correct such phase errors. Signature deconvolution operators can be designed with the multidirectional signatures.
Thus, this method can become more reliable for seismic exploration and reservoir characterization especially in areas that need more vertical resolution.
Reservoir monitoring is another area that may be improved by using deterministic deconvolution with reliable source array signatures.
On board, as survey occurs, real time analysis of the measured signatures can provide to the acquisition a better quality control of the seismic source as well as of the navigation of the vertical cables.
A better way to acquire the multidirectional signature of the source at the time of the seismic data acquisition is, according to the method of the invention, to make use of the technology known as vertical cable.
Vertical cable is a technique used in several applications. For example, U.S. Pat. No. 4,694,435 teaches a vertical device for receiving acoustic waves in water which comprises a tubular element formed of several connected sections, buoyancy means close to a first end, ballasting means close to the second end, several receivers spaced apart inside certain sections of the tubular element and stabilizer fins fixed to the second end thereof. A cable connects the tubular elements to a towing vehicle.
U.S. Pat. No. 4,970,697 teaches a method of acquiring seismic data which includes a horizontal towed receiver array in conjunction with at least one vertically oriented receiver array formed from a plurality of receiver elements spaced apart vertically within a plurality of cable. By simultaneously recording the data from both arrays, seismic data can be acquired for locations directly beneath a fixed obstruction, which a towing vessel has had to steer around. This method is not directed to the measurement of the far-field signature.
Also, U.S. Pat. No. 5,029,145 teaches a method of geophysical exploration whereby shot points and receiver locations are positioned such that seismic data resulting therefrom can advantageously be processed employing 3-D processing techniques to obtain a better image of the earth's subsurface structure. According to the described technique, the vertical cable is used to improve the 3-D image of the earth's subsurface. No far-field signature measurement is sought in the described technique.
The signatures can be used to compute deconvolution operators to be applied on the seismic data. This procedure will improve the resolution of the final seismic image compared with the images obtained from conventional acquisitions, which do not have true signature available. Because of the 3D nature of the VC technique, signatures with different incident angles and azimuths (multidirectional) may be collected.
Therefore, the open literature is devoid of a method able to directly or indirectly measure the multidirectional far-field signatures from the seismic source array with exactly the same acquisition parameters and sea conditions in which the seismic data was acquired, such method being described and claimed in the present application.